1. Field of the Invention
This invention relates to a photo-conductive film and an electrophoto-graphic photosensitive member.
2. Description of the Prior Art
Heretofore, as an electrophotographic photosensitive member composed of an inorganic photoconductive material, there have been widely used selenium, cadmium sulfide, zinc oxide and the like.
On the other hand, as an electrophoto-graphic photosensitive member composed of an organic photoconductive material, there have been used electrophotographic photosensitive member composed of a photoconductive polymer such as poly-N-vinylcarbazol and the like, a low molecular weight organic photoconductive material such as 2,5-bis(p-diethylamino-phenyl)-1,3,4-oxadiazole and the like, a combination of the organic photoconductive material and various dyes and pigments, or the like.
Electrophotographic photosensitive members composed of organic photoconductive materials have advantages such as good film shapeability, capability of being produced by coating, very high productivity, and low cost. In addition, color sensitivity can be advantageously optionally controlled by selecting appropriately a sensitizer such as dyes, pigments and the like, and the investigation has been widely made.
In particular, there have been recently developed a photosensitive member of function separation type which comprises a charge generation layer composed of an organic photoconductive pigment and a so-called charge transport layer composed of the above-mentioned photoconductive polymer, low molecular weight organic photoconductive material or the like and therefore, the sensitivity and durability have been remarkably improved while conventional organic electrophotographic photosensitive members have only low sensitivity and durability, and the organic electrophotographic photosensitive members have recently become to be practically used. Furthermore, various compounds and pigments suitable for photosensitive members of function separation type have been found.
Since the photosensitive member of function separation type is composed of at least two layers, that is, a charge generation layer and a charge transport layer, charge carriers produced by light absorption at the charge generation layer is injected into the charge transport layer to eliminate the surface charge of the photosensitive member resulting in formation of electrostatic contrast. The role played by the charge generation layer in the above-mentioned process is very important. In other words, how many charge carriers are uniformly formed, how effectively the resulting charge carriers are injected into the charge transport layer, and how smoothly the reverse charge carriers flow into the support, in other words, most of electrophotographic characteristics such as electrostatic characteristics, image characteristics and the like, are dependent on the charge generation layer to a great extent.
In general, it is considered that electrophotographic characteristics become better according as the charge generation layer is more uniform, thinner and smoother, that is, the charge generation particles are finer. Therefore, the biggest problem in the practical use is how to produce stably a very thin layer. In order to solve this problem, there should be solved a problem of dispersibility, i.e. how to disperse in a fine powder form the pigments or dyes which are charge generation materials and a problem of dispersion stability, i.e. how to produce a dispersion liquid which is free from aggregation and stable.
As a method for producing a dispersion liquid of pigment or dye in a form of fine particles, in general, pigment or dye together with a binder is ground by a sand mill, ball mill, roll mill or attritor, and if the dispersion conditions are optimized, it is possible to pulverize to a considerable level. In addition, when pigment or dye is formed in a fine particle form in the synthesis and purification stages and the fine particle form is maintained by a certain technique, it is not difficult to prepare a dispersion liquid containing fine particles less than 0.1 .mu.m in size of pigment or dye.
However, in general, the finer the pigment or dye particles, the larger the factors hindering the stability of coating liquid such as aggregation, thixotropy and the like. Thus, dispersibility often contradicts dispersion stability.
As to dispersion stability, in the field of paints which has an old history, there are almost established techniques such as using SP value for matching pigment or dye, binder and solvent, adding an additive, so-called dispersion stabilizer, and the like. On the contrary, there has not yet been established any technique capable of stabilizing a dispersion liquid of finer particles of pigment or dye for electrophotography, but the technique is still in a state of so-called "groping in the dark".